Typical building fire alarm systems include a number of fire detectors positioned throughout a building. Signals from those detectors are monitored by a system controller, which, upon sensing an alarm condition, sounds audible alarms throughout the building. Flashing light strobes may also be positioned throughout the building to provide a visual alarm indication. A number of notification appliances comprising audible alarms and strobes, the audible alarms and strobes being generally referred to as notification devices, are typically connected across common power lines on a notification circuit.
A first polarity DC voltage may be applied across the notification circuit in a supervisory mode of operation. In this supervisory mode, rectifiers at the notification appliances are reverse biased so that the alarms are not energized, but current flows through the power lines at the notification circuit to an end-of-line resistor and back, allowing the condition of those lines to be monitored. Because notification circuits are supervised using an end-of-line resistor, the wires of the circuit must be a single continuous run with no branches and an end-of-line resistor across the wires at the end farthest from the system controller. With an alarm condition, the polarity of the voltage applied across the power lines is reversed to energize all notification appliances on the notification circuit.
U.S. Pat. No. 5,559,492 issued to Stewart et al. (hereinafter the '492 Stewart patent) operates according to the system described above. The '492 Stewart patent further discloses that the visual alarms, or strobes, may be synchronized to fire simultaneously resulting from power interruptions, also referred to as synchronization pulses, in the power lines. Additional timing lines for synchronizing the strobes are not required because the synchronizing signals are applied through the existing common power lines.
Other alarm systems have controlled the function of the audible and visual alarms by interrupting the power signal to the alarms in a predetermined pattern as control signals over the common power lines or by communicating during the synchronization interruption of power. The audible and visual alarms operate their respective loads responsive to the control signal received.
One type of alarm system is an indoor commercial paging system. Common area indoor commercial paging systems (non-fire alarm) have used a “constant voltage” (25, 70 or 100 volt) technology for decades. Briefly, this technology allows easy distributed ceiling or wall speaker design that involve speakers that use “matching transformers” for each and every speaker in the system. These transformers permit easy calculation of how much power is needed for adequate volume in a given area.
For example, if a system consists of twenty speakers, and an adequate power for each speaker is one watt, then the driving power amplifier would have to provide at least twenty watts to adequately handle twenty speakers. Typically, though, it is more desirable to have a larger amplifier, say 50 or 100 watts, to accommodate for speakers that sound in a larger area and may require more than one watt for good sound level coverage.
This is where the transformer comes into play. Commonly, the speaker's transformer has multiple connections, or “taps,” that range from ⅛ watt to as much as 30 or more watts. Again, just adding up the wattage for the system determines the size of the power amplifier that will drive it.
This same speaker technology has generally been adopted for use with audio (voice) fire alarm systems that utilize speakers to alert occupants of a building of an emergency. The warning typically consists of alert tones followed by spoken word messages that give instructions to occupants during the emergency.
The disadvantage to using this technology lies in the setting of each speaker's transformer taps. If it is deemed that a particular speaker is not loud enough in a given area, the service technician must remove the speaker from the wall or ceiling, move the tap connector to the next higher tap setting, re-install the speaker and then test the output, usually with a dB meter, to see if the audio is now loud enough. (NFPA 72 “National Fire Alarm Code” requires that speakers used in fire alarm systems produce a sound that is at least 15 dB above the ambient noise level of a given area).
This is, more or less, a trial-and-error method of setting speaker loudness, and may have to be repeated several times. One of the biggest factors in determining proper dB levels lies in the actual construction material of the area in question, and the anticipated ambient noise level. There are methods to predict the required dB level before installation, but it is cumbersome and expensive to make this prediction, particularly if there are unknowns involved, usually in new construction situations. Thus, these predictive methods are not widely used for fire alarm systems.